(CNN) -- A British security contractor on trial in Iraq for killing two colleagues said Sunday he is "not guilty of murder."

Daniel Fitzsimons, the first Westerner to stand trial in post-Saddam Hussein Iraq, is accused of the 2009 shooting deaths of Paul McGuigan, a British national; and Darren Hoare, an Australian. He is also accused of the attempted murder of a guard.

Asked for a plea, Fitzsimons responded, "not guilty of murder, I am guilty of manslaughter with diminished responsibility."

The court entered his plea as "not guilty."

In dramatic detail, Fitzsimons laid out for the three-judge panel his story of what happened in the August 2009 incident, and described shooting his colleagues in what he believed to be self-defense.

When a judge told him that some of his story did not match evidence, Fitzsimons said the "crime scene is corrupted."

He also complained that the court would not let him talk about his post-traumatic stress disorder. "I do not believe this is a fair trial," he said.

The presiding judge adjourned the case until February 20 to get an opinion from a health committee on PTSD and the effects it can have.

If convicted, Fitzsimons could face the death penalty. Under Iraqi law, murder is punishable by hanging.

The trial follows a U.S.-Iraq security agreement that went into effect in January 2009, ending foreign contractors' immunity. Before that, Iraqis had complained that private security contractors were operating in a state of lawlessness, never held accountable by Iraqi authorities for incidents in which Iraqis were killed. The most famous such case involved a 2007 incident in which 17 Iraqis were killed in a Baghdad square by security guards working for Blackwater.

At Fitzsimons' trial Sunday was Hassan Jaber, an Iraqi lawyer who was wounded in the Blackwater shooting incident. At one point during the proceedings, he whispered, "This is a victory for justice." After the trial, he told CNN he was very happy to see a British citizen being held accountable for a crime in Iraq, calling it a sign of Iraq's sovereignty.

Fitzsimons, 30, was in a pinstripe suit. Though initially calm, he became impassioned as he testified, standing in front of the three-judge panel.

He said he is a former British soldier who served in the Parachute Regiment and was discharged for mental illness. He was later employed by various security companies in Iraq since 2005. He returned to the United Kingdom, then went back to Iraq in 2009, and was there for just one day when the incident took place.

He and the men he killed were employees of the security firm ArmourGroup, based in the UK.

Fitzsimons said he and a colleague purchased two bottles of whiskey and were drinking in a caravan when a former British marine -- who turned out to be McGuigan -- came into the caravan drunk. He and McGuigan began argue based on an old rivalry between the Marines and the Parachute Regiment. The argument quickly grew when McGuigan allegedly insulted two of Fitzsimons' friends who died in Iraq. Fitzsimons said McGuigan threw the first punch.

The two fought on and off for an hour but then shook hands to end it, Fitzsimons testified. "I had enough ... I took my bottle and went to the caravan next door."

Fitzsimons said he finished the bottle of whiskey and fell asleep. Then, McGuigan and the Australian security guard -- later identified as Darren Hoare -- "crash opened" his door, Fitzsimons said. Before he could open his eyes, he was "booted in the face," "punched" and was "jumped on by both," Fitzsimons testified. He said the two "tried to stomp on me."

Fitzsimons asked the court to check with police about his physical state when he was arrested.

"My head was like a football," he said, describing his eyes being affected and "the front of my body was covered with sandal marks."

He said Hoare pinned him down on the floor and McGuigan grabbed Fitzsimons' company-issued M4 assault rifle, cocked it and pointed it at him."The Australian was holding me down ... laughing," he said.

Fitzsimons then asked the judge to excuse his language and apologized repeatedly for saying the words he was about to share. He said McGuigan said to him, "I am going to (expletive) kill you, you little (expletive)."

Fitzsimons alleged that McGuigan kept repeating the threat as the rifle was pointed at him "locked and loaded." Fitzsimons insisted that he shouted at McGuigan twice, "Stop! Put the weapon down," and that "the Australian" was holding him down.

Fitzsimons managed to grab his company-issued Glock 17 9mm pistol from his body armor and, fighting off Hoare with his other hand, shot McGuigan, he said. "I'm a trained soldier sir," he told the judge. "If anyone points a weapon at you ... to kill ... you respond."

Fitzsimons said he "made a decision in a split-second. Motioning with the gun for the courtroom, he said he "shot him twice in the chest ... I fired two rounds ... bang bang in his chest."

He shot McGuigan a third time in the face before wrestling with Hoare for the pistol, he said. Hoare died during the struggle for the Glock, he said. Fitzsimons said Hoare threatened him, "I am going to (expletive) kill you for killing my friend."

"I made a decision a decision -- I pulled the trigger," Fitzsimons told the presiding judge.

Fitzsimons said he does not remember if he shot two or three rounds into Hoare. "I know one was in the chest ... I think also in his neck or his face," Fitzsimons said.

Later, the judge said the story contradicts coronary reports that indicate Hoare was shot from a distance.

He said he was covered in blood, carrying the pistol as he ran outside in an attempt to get to the British Embassy. He said he was running in the compound, knocking on doors looking for people to help him, but no one responded. "All I could think: This is not safe, police would come ... I am covered in blood, my blood, Darren's blood, Paul's blood."

Fitzsimons said an Iraqi guard working for ArmorGroup -- identified as Arkan Mahdi by the court -- came out of a guard area with a Kalashnikov AK47 rifle and yelled at him to stop. He testified that he stopped and saw the safety off the rifle. "He was confused ... He was not my enemy ... I quickly shot him in the leg ... I had to get to the embassy," Fitzsimons testified.

Mahdi sat in the first row of the small courthouse staring at his shooter and leaning on his crutch.

Speaking through a translator, Fitzsimons, who kept repeating certain sentences, such as, "I had to get to the embassy," got irritated a few times when there were translation issues and muttered "This is so hard."

When the judge told Fitzsimons he contradicted the coroners' reports with his testimony, he responded, "I disagree with those reports ... This is what happened in my head ... It happened as I told you ... It happened as I said it did."

He added, "I question those reports ... That crime scene is corrupted in every way." He accused members of the company of "corrupting the scene," saying, "Bodies were moved by members of the company."

Fitzsimons alleged that after he was detained, he was handcuffed for five days and left outdoors under the sun for the first day. "This was the worst thing that has happened to me in my life and I have been through (expletive) and bad things," he said.

He added that he was "drunk out of my head" during the incident.

Fitzsimons grew frustrated when the judge refused to go into the details of his PTSD, saying that he was diagnosed by three "top psychiatrists" in the UK. The judge said a medical panel had already examined his mental state and ruled that he was "responsible" for his actions. A medical appeals panel certified that decision last month.

"This is not fair," Fitzsimons said. "I do not believe this is a fair trial."

The judge asked Fitzsimons if he had anything to add. "I wish it didn't happen," Fitzsimons said. "Not a day goes by I don't wish to turn back time ... What started as a minor drunken argument turned into a major fight and tragic consequences."

An Iraqi lawyer for McGuigan and Hoare addressed the court, asking for the death penalty. The prosecutor said there is enough evidence to convict Fitzsimons.

Fitzsimons' lawyer Tariq Harb, head of Iraq's Bar Association, asked that his client's PTSD and mental illness be taken into consideration, as well as the fact that he was under the effects of alcohol. He asked for a reduced sentence.

In a statement at the time of the incident in 2009, ArmorGroup Iraq, part of parent company G4S, said it was "working closely with the Iraqi authorities to investigate the circumstances."

Amnesty International has expressed concern about the case. In a statement in August, the group's UK Campaigns Director Tim Hancock said that while contractors should be held "fully responsible" for alleged wrongdoing, "Iraq has an appalling record of unfair capital trials and there's a definite danger of Danny Fitzsimons being sentenced to death after a shoddy judicial process."

Iraq is one of the biggest users of the death penalty in the world, Amnesty noted.